Facebook posted a slower revenue growth in the first quarter of 2015 with a profit of $512 million, down 20% on a year earlier.

Revenue rose 42% to $3.5 billion, a figure slightly below analysts’ forecasts.

A bright spot was the rise in monthly active users, up 13% from a year earlier to 1.44 billion.

Notably, for those investors concerned about the company’s efforts to appeal to younger users who access Facebook on their smartphones, monthly mobile users increased by 24% to 1.25 billion, a majority of the site’s users.

Facebook has been particularly adept at channeling that growing mobile user base into advertising dollars. The company said that during the quarter, revenue from mobile ad sales made up nearly three-quarters of total ad sales.

Investors have been worried about slowing revenue growth, as well as increasing costs at the company. Facebook has been spending more on research and development as it moves beyond its original social networking operation.

Spending on research and development jumped to $566 million from $181 million a year earlier.

Facebook has warned that those costs are set to increase, as it looks to expand some of its acquisitions including photo-sharing site Instagram, messaging service WhatsApp, and virtual reality company Oculus Rift.